In the recent electric power field, great attention has been paid to magneto-optical sensors which contain magneto-optical materials, and are suitable for detecting abnormal currents in power transmission lines which may be caused by thunder-bolts or the like. The sensors can detect magnetic fields generated around power transmission lines utilizing the Faraday effect, which is a kind of magneto-optical effect. The detection is carried out utilizing the fact that the Faraday rotational angle is changed depending on the intensity of a magnetic field. It is generally known that when a magneto-optical element having the Faraday effect is irradiated with a laser beam, and a magnetic field is generated in the same direction as the propagation direction of the laser beam, the polarization plane of the incident laser beam is rotated in proportion to the intensity of the magnetic field. Polarizing plates having different polarization planes are arranged on the front and back sides in the light propagation direction of the magneto-optical element, utilizing the above-described rotation of the polarization plane. Accordingly, the difference between the rotational angles of the polarization planes causes the difference between the light-quantities to appear. The difference between the light-quantities is detected by a light-sensing means such as a photodetector or the like. Thus, the strength of abnormal current can be detected. Magneto-optical sensors using the Faraday effect as described above have a high sensitivity. Moreover, the sizes and weights can be reduced. Furthermore, the explosion-proof performance is high, and the sensors can be remote-controlled. Also, since optical fibers are used for propagation of light, the electromagnetic induction noise levels and the insulating properties are superior. Thus, the magneto-optical sensors have superior characteristics compared to electrical type magnetic field sensors.
Referring to the characteristics of paramagnetic materials to form magneto-optical elements, it has been required that the Verdet constant (V: (deg/(Oe·cm)) is high. The Verdet constant means a Faraday rotational angle per unit length and per unit applied magnetic field. The Verdet constant has a relationship represented by θf=VHd, in which θf represents a Faraday rotational angle (i.e., the angle of a polarized light beam), d represents a movement distance of the light beam which passes through the magneto-optical element, and H represents the intensity of a magnetic field applied to the magneto-optical element. According, the change ratio of the Faraday rotational angle increases as the Verdet constant becomes larger. Thus, the difference between light-quantities increases when the magnetic field is slightly changed. Thus, a magneto-optical sensor having a high sensitivity can be provided.
As a magnetic material having the above-described properties, a single crystal having an yttrium iron garnet structure (Y3Fe5O12: hereinafter, referred to as YIG for short) is used as a ferromagnetic material described in Japanese Examined Patent Application Publication No. 2-3173. The YIG single crystal is advantageous in that the Verdet constant is large, and the sensitivity to magnetic variation is high. However, the YIG single crystal described in Japanese Examined Patent Application Publication No. 2-3173 has the following problems: the Faraday rotational angle increases until the intensity of a magnetic field reaches a predetermined value, and then becomes constant after the intensity reaches the predetermined value, i.e., the Faraday rotational angle becomes magnetically saturated. Therefore, when the YIG single crystal is used as a magneto-optical element of a magneto-optical sensor for detection of large current, problems occur in that the sensor can not accurately detect electric current. Also, the YIG single crystal can transmit only light rays in an infrared range of 1000 nm to 5000 nm. Thus, problematically, the YIG single crystal cannot be used in a visible range of 400 nm to 600 nm and at a wavelength of 650 nm. The wavelength of 650 nm is in the wavelength range for plastic fibers, which has been investigated for use in LAN or the like which is mounted on cars. Light sources for use in an infrared range are expensive. On the other hand, light sources in a visible range are inexpensive. Thus, it is desired to realize a paramagnetic material which can be used in a visible range.
As a magnetic material which can solve the above-described problems, a terbium .aluminum type paramagnetic garnet single crystal (Tb3Al5O12; hereinafter, referred to as TAG single crystal for short) containing at least Tb and Al is described, for example, by S. Ganschow, D. Klimm, P. Reiche and R. Uecker; Cryst. Res. Technol., 34 (1999) pp. 615–619. The Verdet constant of the TAG single crystal is very large compared to the Verdet constants of other paramagnetic materials. Thus, even if the size of the single crystal is reduced, a sufficient Faraday rotational angle can be obtained. Accordingly, the sizes of magneto-optical elements can be reduced. Furthermore, even if a strong magnetic field is applied, no magnetic saturation occurs in contrast to the YIG single crystal. Therefore, the TAG single crystal can be used as a magneto-optical element of a magneto-optical device for detection of large electric current. Hence, the TAG single crystal can detect a wide range of magnetic field intensity. The TAG single crystal has a very high light transmission factor in a light wavelength range of 500 nm to 1400 nm. In addition, it has been revealed that the TAG single crystal can be provided with a high light transmission factor in a visible range of 400 nm to 700 nm in wavelength. It has also been suggested to use the TAG single crystal having the above-described superior properties in magneto-optical devices.
Although the TAG single crystal has the above-described superior properties, no TAG single crystals with a size large enough to be practically applied in magneto-optical devices have been realized. The reason for this is that the TAG single crystal is a decomposition-melting type compound. Thus, the composition of starting raw materials obtained when the materials are melted is different from the composition of a crystal obtained when the melted raw materials are cooled. More specifically, the decomposition-melting type TAG single crystal composed of a garnet phase cannot be obtained directly from the composition of the melted starting raw materials. Thus, problems occur in that TbAlO3 composed of a perovskite phase is mixed with the TAG single crystal. The TAG single crystal has the largest Verdet constant of the paramagnetic dielectrics. However, the Verdet constant of the TAG single crystal when it is irradiated with a light beam with a wavelength of 633 nm is about 0.01°/(Oe·cm). Thus, for application of the TAG crystal for magneto-optical devices, the TAG single crystal is required to have a larger Verdet constant.
As another terbium type paramagnetic garnet single crystal, for example, terbium.gallium.garnet (Tb3Ga3O12: hereinafter, referred to as TGG single crystal for short) or the like is known. However, the Verdet constant must be increased similar to that of the TAG single crystal. The TGG single crystal is a coincidently melting type material. That is, the composition of the starting raw materials of the TGG single crystal is the same as that obtained after the crystal is grown. Thus, the TGG single crystal having a practical size can be easily produced by the known Czochralski process. However, the obtained TGG single crystal has problems in that the Verdet constant at a wavelength of 633 nm is small, i.e., 0.0075°/(Oe·cm). Magneto-optical materials which have a large Verdet constant and are effective in size-reduction are desired for the decomposition-melting type TAG single crystal and also coincidently melting type terbium type paramagnetic garnet materials.
It is an object of the present invention to solve the above-described problems and to provide a terbium type paramagnetic garnet single crystal of which the Faraday effect is large, the light transmission factor is high, and the Verdet constant is enhanced, and to provide a magneto-optical device using the terbium type paramagnetic single crystal.